Installed electrical power equipment is often subjected to diagnostic tests, such as partial discharge tests, requiring high alternating voltage at power frequency. Regular power frequency voltage sources conventionally used in factories for high voltage tests typically include relatively heavy and bulky transformers that are not practical for use in the field where installed equipment needs to be tested. Fortunately, when the electrical equipment to be tested can be considered as an electrical capacitor, as is the case for an electrical power cable, it is possible to generate a desired test voltage using a resonant transformer (also referred to herein as “reactor”), which is often significantly smaller and lighter than a regular transformer of the same rating, and can be transportable to sites outside the factory. If L is the inductance of the resonant transformer, C is the capacitance of the cable to be tested, and f is the frequency of a voltage source input to the resonant transformer, resonance is said to be achieved when 2πfL=1/(2πfC). Under resonance conditions, the voltage across the test cable becomes a large multiple, Q, of that of the alternating voltage source, where Q is referred to as the quality of the circuit. Thus, starting with a modest voltage magnitude (e.g., ˜1 kV), it is possible to generate a high voltage (e.g., ˜25 kV to ˜75 kV).
The capacitance of a given type of cable to be tested varies proportionally with the length of the cable. That is, as the length of a given cable type increases, the capacitance of the cable increases proportionally. In order to achieve resonance for different length cables, either inductance L or frequency f or both have to be adjusted accordingly. In some conventional systems, the resonant transformer can be configured to generate a variable inductance L to achieve a desired inductance value for a given capacitance to be tested.
To achieve a variable inductance resonant transformer, a high voltage winding, sometimes split over two coils, is built around one or two legs of a magnetic core that is split to form two U-shaped magnetic paths facing each other across open air gaps. While one of these U-shaped cores is stationary, the other is connected to mechanical actuators which allow the gap to open or close. The entire assembly, including the mechanical actuators, are generally housed in a relatively large metal tank, normally filled with insulating oil to provide dielectric stability to the coil of the resonant transformer (e.g., to inhibit undesirable discharges in the coil). The forces of electromagnetic origin on the faces of the core across air gaps tend to dictate mechanical and structural designs which result in heavy core assemblies. When testing objects of small capacitance, such as short cables, with commercially available variable inductance transformers, resonance is often not achievable and complex schemes have to be developed to accommodate small capacitance. For example, in one conventional scheme, the resonant transformer can be connected in parallel with the cable, while allowing the resonant transformer to function as an auto-transformer. Furthermore, with cables of large capacitance, the air gap is often forced to assume large values (e.g., approximately 15 cm or more). As a result, the tank housing of the resonant transformer typically must be much larger (and heavier) than desired to accommodate movement of the split magnetic core. Another major contribution to the weight of such transformers is the insulating oil used in the tank.